﻿/*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*      http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

using System;
namespace com.google.zxing.common.reedsolomon
{

    /// <summary> <p>This class contains utility methods for performing mathematical operations over
    /// the Galois Field GF(256). Operations use the primitive polynomial
    /// x^8 + x^4 + x^3 + x^2 + 1 in calculations.</p>
    /// 
    /// <p>Throughout this package, elements of GF(256) are represented as an <code>int</code>
    /// for convenience and speed (but at the cost of memory).
    /// Only the bottom 8 bits are really used.</p>
    /// 
    /// </summary>
    /// <author>  srowen@google.com (Sean Owen)
    /// </author>
    public sealed class GF256
    { 
          public static GF256 QR_CODE_FIELD = new GF256(0x011D); // x^8 + x^4 + x^3 + x^2 + 1
          public static GF256 DATA_MATRIX_FIELD = new GF256(0x012D); // x^8 + x^5 + x^3 + x^2 + 1

          private int[] expTable;
          private int[] logTable;
          private GF256Poly zero;
          private GF256Poly one;

          /**
           * Create a representation of GF(256) using the given primitive polynomial.
           *
           * @param primitive irreducible polynomial whose coefficients are represented by
           *  the bits of an int, where the least-significant bit represents the constant
           *  coefficient
           */
          private GF256(int primitive) {
            expTable = new int[256];
            logTable = new int[256];
            int x = 1;
            for (int i = 0; i < 256; i++) {
              expTable[i] = x;
              x <<= 1; // x = x * 2; we're assuming the generator alpha is 2
              if (x >= 0x100) {
                x ^= primitive;
              }
            }
            for (int i = 0; i < 255; i++) {
              logTable[expTable[i]] = i;
            }
            // logTable[0] == 0 but this should never be used
            zero = new GF256Poly(this, new int[]{0});
            one = new GF256Poly(this, new int[]{1});
          }

          public GF256Poly getZero() {
            return zero;
          }

          public GF256Poly getOne()
          {
            return one;
          }

          /**
           * @return the monomial representing coefficient * x^degree
           */
          public GF256Poly buildMonomial(int degree, int coefficient)
          {
            if (degree < 0) {
              throw new ArgumentException();
            }
            if (coefficient == 0) {
              return zero;
            }
            int[] coefficients = new int[degree + 1];
            coefficients[0] = coefficient;
            return new GF256Poly(this, coefficients);
          }

          /**
           * Implements both addition and subtraction -- they are the same in GF(256).
           *
           * @return sum/difference of a and b
           */
          public static int addOrSubtract(int a, int b) {
            return a ^ b;
          }

          /**
           * @return 2 to the power of a in GF(256)
           */
          public int exp(int a)
          {
            return expTable[a];
          }

          /**
           * @return base 2 log of a in GF(256)
           */
          public int log(int a)
          {
            if (a == 0) {
              throw new ArgumentException();
            }
            return logTable[a];
          }

          /**
           * @return multiplicative inverse of a
           */
          public int inverse(int a)
          {
            if (a == 0) {
              throw new ArithmeticException();
            }
            return expTable[255 - logTable[a]];
          }

          /**
           * @param a
           * @param b
           * @return product of a and b in GF(256)
           */
          public int multiply(int a, int b)
          {
            if (a == 0 || b == 0) {
              return 0;
            }
            if (a == 1) {
              return b;
            }
            if (b == 1) {
              return a;
            }
            return expTable[(logTable[a] + logTable[b]) % 255];
          }
    
    
    }
}